Rotor Frequency and Speed

Correct Answer

B — 870 rpm

Concept and Reasoning

Rotor frequency is slip times supply frequency. Thus s=2/60=1/30. For 8 poles at 60 Hz, Ns=900 rpm and N=(1−1/30)×900=870 rpm.

Calculation / Key Relation

s=fr/f=2/60=1/30

Ns=120f/P=120×60/8=900 rpm

N=(1−s)Ns=(29/30)×900=870 rpm

Exam Takeaway

Rotor frequency directly gives slip: fr=sf.

Common Mistake

Do not use rotor frequency itself in the synchronous-speed formula.